# High availability load balancing (HALB)

High availability load balancing (HALB) in Parallels RAS is a functionality that load balances RAS Secure Gateways. The load balancer is built into a Parallels HALB appliance, which is a preconfigured virtual machine with the operating system installed and all relevant settings configured.

Parallels HALB appliance has been tested to work for the following hypervisors:

* Microsoft Hyper-V
* VMware ESXi
* Nutanix AHV
* Scale HyperCore

Please note that other hypervisors may also be used, and support will be provided, but only on a best effort basis.

HALB is deployed in Parallels RAS on a Site level. You can have multiple HALB configurations per Site, which are called Virtual Servers. Each Virtual Server has its own IP address (called Virtual IP or VIP) and is assigned one or more HALB appliances (also called HALB devices in the Virtual Server context) that perform the actual load balancing. An HALB Virtual Server is a virtual representation of HALB devices. It provides traffic distribution to HALB devices when they are properly configured. Since the IP address of a specific Virtual Server is the single point of contact for the client software, it is recommended to have at least two HALB devices per Virtual Server for redundancy.

Multiple HALB devices assigned to a Virtual Server can run simultaneously, one acting as the primary and others as secondary. The more HALB devices a Virtual Server has, the lower the probability that end users will experience downtime. The Virtual Server is assigned the IP address of the primary HALB device, which is shared with secondary HALB devices. Should the primary HALB device fail, a secondary is promoted to primary and takes its place using the same IP address for client connections.

{% hint style="info" %}
**Note:** Please note that when a secondary HALB device is promoted to primary, a user may experience up to two disconnects. The first disconnect will occur when an HALB device goes down. The second disconnect may happen when a device goes back online. The disconnects cannot be avoided because the virtual IP address has to be transferred from one HALB device to the other, which means that the first device has to stop communications over this address, while the other device will have start it. Note that disconnects don't affect user sessions. Users are able to reconnect to their sessions and no user data is lost.
{% endhint %}

Setting up High Availability Load Balancing consists of the following steps:

1. Deploying one or more Parallels HALB appliances (devices).
2. Configuring one or more Virtual Servers in the RAS Console.

Read on to learn how to download and deploy a Parallels HALB appliance.


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